1. Field of the Invention
The present invention relates generally to a data recording/playback apparatus and method for use in a system for recording and playing back digital signals with high density, and, more particularly, to a data recording/playback apparatus and method for an overwrite medium which is susceptible to errors resulting from residual marks, for example in an optical recording medium.
2. Description of the Related Art
In a conventional digital data recording/playback apparatus, a fixed data identification level is set as a threshold for establishing bit levels of 1 or 0 for an analog playback signal. For example, Japanese Patent Laid-Open No. 2-81324 discloses a method in which the center of the envelope of a playback signal is sliced.
For an optical recording medium capable of being overwritten, and a magnetic recording medium useful for optomagnetic recording and phase change recording, unevenness in recording sensitivity may develop due to the nature of the processing medium, which causes the level of the playback signal to fluctuate. For example, the typical data detection method suffers from low-frequency AC modulation distortion due to reflectivity variations between individual tracks on an optical disk. Consequently, a fixed data identification level has proven difficult to establish. The conventional practice has thus been to eliminate the variation of the signal level or to devise circuitry in which the identification level itself follows the fluctuation of the playback signal level.
Further, as optical bits on the order of microns in size are used for recording and playing back data on optomagnetic and phase change recording media, high-density recording has become possible, as compared with magnetic recording. However, the residual mark per bit unit tends to increase for the thermal recording that is commonly employed for optical systems. Thus, detection errors originating from the level fluctuation per bit unit due to residual marks are likely to occur even though the data identification level is properly set by the digitizing circuitry.